Burner control apparatus



Dec. 14, 1965 R. w. BROWN 3,223,133

BURNER CONTROL APPARATUS Filed Jan. 2, 1964 s Sheets-Sheet 1 E. Q: m X x L m H QNI Nam N A N mi a WW) .1 l. w A a; .1. NW F EN w QN Mm 4 mm mw M M ff B W 2 6m oznmqwa Dec. 14, 19.65 R. w. BROWN BURNER CONTROL APPARATUS 3 Sheets-Sheet 2 Filed Jan. 2, 1964 low KMEC. clxl. v Q:

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BURNER CONTROL APPARATUS Dec. 14, 1965 Filed Jan. 2, 1964 3Sheets-Sheet 5 FIGS L2, 8 2A TIME IN SECONDS 95' 95' PRESSURE SWITCH TB 5 OMITTED 2R5 JZ Tl4 T 111 INVENTOR. Paw/42p M Pww/v BY FIE 2a ATTOP/UEV United States Patent 3,223,138 BURNER CONTRGL APPARATUS Richard W. Brown, Excelsior, Minn, assignor to Honeywell Inc., a corporation of Delaware Filed Jan. 2, 1964, Ser. No. 335,106 17 Claims. (Cl. 158-28) The present invention is concerned with an improved burner control apparatus and more particularly with a burner control apparatus of the type wherein a timer is utilized to provide a starting sequence of operation of the burner system to safely bring the system from a stand-by condition to a run condition when there is a demand for heat from the burner, and to provide a shutdown sequence of operation to bring the system from the run condition to the stand-by condition when this demand for heat is satisfied.

Specifically this invention is concerned with a burner control system as described above wherein there is provided means for detecting a malfunction of the system wherein the main fuel control valve becomes stuck open or only partially closed during a period when the fuel supply to the burner should be completely shut off, and further provides means operable upon the occurrence of such a malfunction for disabling the system in such a manner that it cannot be set back in operation except by one thoroughly skilled in the operation of these systems and the dangers resulting from such a malfunction.

Further the invention includes a provision of similar means for disabling the system upon an indication by the combustion sensing apparatus of the existence of combustion during certain periods when no combustion should be present at the burner. In addition the invention provides an alarm system operable only upon the occurrence of such malfunctions to call attention thereto.

In the burner control art it is well known to provide a control system wherein a timer provides a starting sequence of operation of the burner components to safely bring the system from stand-by to run condition. In so doing, the timer may provide a pre-purge period during which combustion air is provided to purge the fire box of any accumulation of combustible fuel and to thereby prevent the possibility of an explosion when an attempt is made to establish combustion at the burner. Following the pro-purge period may be a trial for pilot ignition period wherein fuel is supplied to a pilot burner and the igniter is energized.

It is also known to provide, in series with the energizing circuits for the pilot fuel valve and the main fuel, the parallel combination of a normally open flame switch and a timer operated switch. The flame switch is adapted to be closed by a combustion sensing apparatus, such as a unit including a photo-cell or flame rod for detecting combustion in the fire box, whenever such combustion is present. The timer operated switch is closed at the beginning of the ignition period but is opened a predetermined time after initiation of this period and remains open during the run period. Thus if combustion is not sensed prior to opening of the timer switch or if combustion fails thereafter, the circuit to the pilot and main valve is interrupted.

It is also common to provide, as an additional safety device, a manually resettable safety cut-out device, such as one including a bimetal and a heater therefor, which is effective to lock out the system a predetermined time after the initiation of the ignition period. The combustion sensing apparatus is effective to de-energize the heater for the safety cut-out means upon the establishment of combustion. When combustion at the pilot burner has been safely established, the timer may bring about a main burner ignition period wherein fuel is supplied to the main burner as well as the pilot burner and during this period combustion is normally established at the main burner. During this period the combustion sensing means is effective to shutdown the system in the absence of combustion in the fire box. At the end of this main burner ignition period the pilot burner and the igniter are normally de-energized and, if the combustion sensing means then detects combustion in the fire box, the-system continues into a run period. The system then operates under the supervision of the combustion sensing means as long as there is a demand for heat. Should a flam failure occur during this run period, the fuel supply is immediately interrupted due to the action of the combustion sensing means. When the demand for heat is satisfied, the timer returns the system to a stand-by condition This final portion of the time cycle may include a post-purge period wherein the blower is operated to purge the fire box of any unburned fuel which may have accumulated there after combustion is terminated, and hence avoid the creation of a hazardous condition due to the accumulation of such fuel.

Normally such systems function properly and during the life of a system it is operated through many cycles of operation similar to that described above without malfunction of any of the components. It has long been realized, however, that, should a malfunction occur in such a system, very dangerous conditions may result which present a serious danger to life and property. Therefore, in the past, many attempts have been made to add safety features to these systems which were effective to prevent a hazardous condition in the remote possibility that a malfunction occurs in any of the system components. One of these safety features is the provision of the resettable safety cut-out means which was described above and which is effective to lock-out the system in case combustion is not established within a predetermined time after opening of the valve which supplies fuel to the pilot burner. It is also customary to provide, in such a system, an arrangement whereby it is essential that the combustion sensing apparatus be in a condition so that it indicates the absence of flame before the burner system can be brought from the stand-by condition to the run condition. Elaborate systems have been provided for continuous component checking to almost immediately reveal a malfunction in the components of the system and particularly in the combustion sensing apparatus. A recent improvement in systems of this type is the provision of a main relay circuit which is such that the main control relay cannot be energized to start up the system unless the circuit is completed through the heater of the safety cut-out means which was described above. This feature is disclosed in the William B. Hamelink Patent 3,082,813 and is utilized in the control with which the present invention is described herein.

One possible malfunction which the prior art has failed to provide means to counteract is the possibility that the main fuel valve should become stuck open or should only partially close at the end of the run period when it is desired that the fuel supply be completely shut off. Should this happen, the fire box would continue to be supplied with fuel and in most instances, combustion would continue until the supply of combustion air was extinguished. At such time a fuel rich flange outage would occur and, if subsequently an attempt is made to restart the system, a violent explosion would in all likelihood occur.

Systems are known wherein the supply of combustion air is maintained as long as the combustion sensing apparatus senses the presence of combustion at the burner. Such a system is shown, for example, in the Frederick E. Lange Patent number 2,493,412. A system of this type has the disadvantage that the combustion is then Patented Dec. 14, 1965 continued and safety controls, such as limit controls, may be completely overridden and extremely hazardous conditions result from over-heating of the unit, bursting of boilers, etc. While a fuel rich fiame outage itself may be hazardous, in many instances it is preferable to risk such an outage rather than to continue operation of the burner and to override all of the safety controls which, at least in an unattended system, will almost certainly result in disastrous consequences.

The present invention provides an improvement over the prior art in that it provides means for detecting failure of the main fuel control valve to be closed during periods when no combustion should be present at the burner and, upon occurrence of such condition, for disabling the system and preventing ready restarting thereof. The invention may take several forms and, broadly, provides means for detecting the presence of fuel downstream of the main fuel valve during a portion of a purge period and, upon occurrence of such condition, for rendering operative an inaccessible lockout or system disabling means. The means for detecting the presence of fuel downstream of the main valve may take the form of a pressure responsive apparatus, responsive to the presence of fuel, and it may also utilize the combustion sensing apparatus itself since in most instances the fuel -valves failure to be closed will result in continued combustion after combustion would normally be extinguished. These two devices may be used singly or in combination in a given system as will be appreciated more fully upon reading the complete description of the invention which appears hereinafter. When the combustion sensing apparatus itself is utilized, the invention also provides the feature that the system is disabled upon malfunction of the combustion sensing apparatus which results in an indication of combustion during a purge period when there should be no combustion at the burner.

As used herein, the term inaccessible, when referring to the system disabling means, refers to a disabling apparatus which is so imbedded in the control that it cannot be reset by one who it not thoroughly skilled in the operation of the system and familiar with the hazards which exist upon the occurrence of such malfunction. Rather than having a conventional reset button, as is usually provided with the safety lockout device described above and normally incorporated in systems of this type, this inaccessible disabling means will normally be located within the case which houses the control and hence requires a skilled technician to actually open the control device before it can be reset and again placed in operation. As described herein, this inaccessible disabling means may be in the form of a lockout relay having a contact in the circuit which supplies power to the control device and to the coil of the relay itself. This relay is normally energized to maintain the contacts closed but is controlled by the before mentioned means which is respnosive to the presence of fuel downstream of the main valve so as to be deenergized when fuel is present downstream of the valve. The relay circuit also includes a timer operated contact which maintains the circuit to the relay coil completed at all times except during portions of the purge period. During these times the inaccessible relay is thus placed under the control of the means for sensing fuel downstream of the valve.

In another embodiment of my invention the inaccessible disabling means takes the form of an inaccessible, excess current responsive, circuit interrupting means such as a fuse or manually resettable circuit breaker connected in the circuit which supplies power to the control apparatus. This device is so chosen that it will not be disabled during normal operation of the system. Upon the presence of fuel downstream of the valve or upon the indication of combustion, as the case may be, a switch is operated which is adapted to complete a circuit through the disabling device so as to connect it directly across the source of power and hence to cause it to be disabled.

This circuit, however, also includes a switch operated by the timer and adapted to be closed by the timer only during a portion of a purge period so that the disabling apparatus can be operated only during that period if the undesired malfunction has occurred.

In each of the embodiments described briefly above, my invention includes the optional provision of an alarm such as a visual or audible alarm, which is energized upon operation of the inaccessible system disabling means. This alarm is separate and distinct from the alarm which is normally associated with the manually resettable safety switch conventionally provided in systems of this type and also retained in the preferred embodiment of my invention.

From the foregoing brief description of my invention and, as will be more cleraly pointed out in the following detailed description thereof, it can be seen that my invention provides additional safety features in a burner control apparatus of the type using a sequencing timer and, when used with the system which has the normal manual resettable safety switch, provides a system which has incorporated therein a dual safety lockout provision. It will be appreciated by those skilled in the burner control art that this invention provides a significant safety feature and therefore an important improvement not found in prior systems of this type.

My invention will become more apparent to those skilled in the art upon reading the following detailed description of two preferred embodiments of the invention in conjunction with the accompanying drawing wherein:

FIGURE 1 is a schematic showing of a burner control apparatus having incorporated therein one form of my invention, and with the apparatus connected in controlling relationship with a typical fuel burner unit.

FIGURE 2 discloses a similar burner control apparatus utilizing another form of my invention and controlling a similar typical fuel burner unit.

FIGURE 2a discloses, in fragmentary schematic form, a modification of my invention as used in connection with the circuit of FIGURE 2.

FIGURE 3 is a bar graph showing the time relationship of the operation of the contacts of a timer which forms a part of the control apparatus of FIGURES 1, 2, and 2a.

In the following description like characters shall be used in reference to similar components in the various figures. In the bar graph of FIGURE 3 brackets have been used to indicate which of the timer contacts are found in each of the several figures of the drawing. This has been done to aid in clarifying the following description.

Referring to FIGURE 1, it will be seen that power is supplied to the control apparatus through a pair of conductors 10 and 11. These conductors are adapted to be connected to a source of power (not shown) such as a conventional volt A.C. source. In some installations it may be desirable to interpose a transformer (not shown) between the normal power supply and the conductors It) and 11 in order to lower or raise the voltage as desired. Conductor 10 has interposed therein a manual switch 12 which may be operated to completely interrupt the supply of power to the system. Conductor 11 is connected to a further conductor 111 through a switching arrangement to be described hereinafter. In the following description of the system it will be noted that all of the system components have assumed the position which they occupy when the switch 12 is open.

The control apparatus is controlled by a start-stop station generally designated at 13. This start-stop station is seen to include a normally open start switch 14 and a normally closed stop switch 15 with these two switches connected in parallel circuit and in turn connected to conductor 10 by a conductor 16. This start-stop station controls the supply of current to a main control relay 1R. Relay IR is seen to have an electrically energizable winding connected in circuit with the start-stop station by a pair of parallel conductors 17 and 18. This winding controls the operation of normally open relay contacts 1R1, 1R2, and IRS, and a normally closed contact 1R4. As seen in the drawing, this winding has an intermediate tap or terminal 19, defining an upper portion Zita and a lower portion 20b of the winding. As will be seen in the drawing and will be further explained hereinafter the lower portion 2% of the winding controls the operation of the contacts 1R1, 1R2, 1R3, and 1R4.

Reference numeral 21 refers to a manually resettable safety switch or lockout device which controls the circuit through the winding of relay 1R. As seen in the drawing, the safety switch includes a pivoted member 22 which is normally urged in a counter-clockwise direction by a biasing means such as spring 23. Member 22 carries a pair of electrical contacts 24 and 25 which cooperate with a pair of fixed contacts 26 and 27 respectively. Fixed contact 27 is connected to winding 26 or relay 1R by conductor 28. Contact 25 is connected to conductor 111 by a conductor 29. Normally restraining member 22 from counter-clockwise rotation and holding it in a position wherein contact 25 engages fixed contact 27, and wherein contacts 24 and 26 are separated, is a bimetallic member 30. Associated with the member 30 is a heater 31 which is connected in series circuit with the lower portion 29b of the 1R relay winding. Bimetallic member 30 is constructed so that as it is heated the lower end thereof warps to the right and, after a predetermined time of heating, this member warps sufiiciently to release member 22 which is then rotated to a position wherein contacts 25 and 27 are separated and contacts 24 and 26 are closed. A manual reset button 32 is provided so that the device can be reset when member 30 has again cooled.

As pointed out above, heater 31 is connected in series with the lower portion 20b of the winding of relay 1R. The relative impedances of this lower portion 29b of the relay winding and of heater 31 are so chosen that with a given line voltage, the current how is such that heater 31 is operatively de-energiz/ed as long as these two elements are connected in series. However when current flows through this circuit relay 1R is energized to operate its contacts to their actuated position. As will be described in more detail hereinafter, when the circuit through the upper portion Zita of the relay winding is connected in parallel circuit with heater 31 by the closing of a contact T5, there results a transformer effect such that heater 31 becomes operatively energized.

Reference numeral 33 generally designates a burner system which has been shown by way of example as an oil fired burner with a ga pilot burner. It is to be understood, however, that the present invention finds application in control apparatus for numerous types of burners including gas fired main burners with gas pilots and with direct ignition type burners. The system includes a main burner 34 and a pilot burner 35. Pilot burner 35 is supplied with a fuel such as gas and the flow of this fuel is controlled by a pilot valve 36. Main burner 34 is supplied with fuel from a source (not shown) and, where an oil burner is used as in the illustrative case, this fuel is pumped to the burner by a pump 37 which may be of any conventional design and preferably with the by-pass feature normally incorporated in pumps of this type. Pump 37 is operated by a motor 38 which may also supply power to operate a blower 39 for supplying combustion air to the main burner. Downstream of the pump 37 is a main fuel control valve 40. An igniter 41 is adapted to ignite fuel at pilot burner 35 upon energization of an ignition transformer 42.

Reference numeral 46 generally refers to a flame detector which may be of conventional design, for example similar to that shown in the Richard S. Feigal Patent Number 2,556,961. Flame detector 46 has a pair of input terminals one of which is connected to conductor it) by a conductor 47 and the other of which is connected to a conductor 50 which is in turn connected to a conductor 51 which is connected to conductor 11. Input signals are received by flame detector 46 through a pair of conductors 52 and 53 which are connected to a combustion responsive device such as a photocell 54. Photocell 54 is positioned so that it sights through the wall of the fire box and is aligned to sense combustion at either the main burner 34 or the pilot burner 35. It is to be understood that other types of combustion sensing devices may be used in place of photocell 54 and these may include flame rods, bimetal operated contacts or other similarly well known devices. Flame detector 46 also includes a relay 4R which is energized by the flame detector upon the sensing of combustion by photocell 54. Relay 4R operates a normally open contact 4R1 which controls energization of a secondary flame relay ZR. The winding of relay 2R is connected between conductors 1i) and 111 by a conductor 55 and in series with the winding of 2R is the normally open 4R1 contact and the parallel combination of a switch T7 and a normally open contact 2R2 which is operated upon energization of the winding of relay 2R. Relay 2R includes, in addition to the normally open contact 2R2, a normally closed contact 2R1 which is connected in series with heater 31, a normally open contact 2R3, and another normally closed contact 2R4.

Connected across conductors 10 and 111 by a further conductor 56 is a timer designated T. In series with the actuator of timer T is the normally open contact 1R3 and two switches T4 and T10. In shunt with the switch T4 is a manually operable switch 57. Timer T is preferably of the type having an electric rotary motor which drives a plurality of switch operating cams. Timer T operates switches T1 through T12 and in the preferred embodiment of my invention the timer is constructed so that the switches operate on a time cycle substantially as shown in FIGURE 3. In FIGURE 3 the shaded portions of the bar graph indicate portions of the time cycle wherein a given one of the switches is closed and the unshaded portions indicate that portion of the cycle wherein a given switch is open.

Also connected across conductors 1t) and 111 by a conductor 60 is a relay 3R having its winding connected in series with a timer operated switch T1. Connected between conductors 56 and 6t and hence connecting the winding of relay 3R is parallel with timer motor T when switch T1 is opened, is the normally closed contact 1R4. Relay 3R operates a normally open contact 3R1 which controls energization of motor 38 of the burner apparatus. Contact 3R1 is interposed in a conductor 61 which connects one terminal of motor 38 to conductor 10. Another terminal of the motor is connected to conductor 51 by a conductor 62. As pointed out previously, conductor 51 is in turn connected to conductor 11.

Also adapted to be connected across conductors 10 and 11 is an alarm 62 which may be a conventional audible or visible alarm. One terminal of this alarm is connected to conductor 10 by a conductor 63 and the other terminal is connected to fixed contact 26 of the safety switch by a conductor 64. Contact 24 mounted on movable member 22 of the safety switch, and which is adapted to electrically engage contact 26 when the safety switch is operated, is connected to conductor 51 by a conductor 65. Thus when the safety switch is operated a circuit will be completed through alarm 62.

As seen in the drawing, main valve 4% is adapted to be connected across conductors 10 and 11 through conductor 16, start-stop station 13, a conductor 66 which has interposed therein normally open contact 1R2, the parallel combination of contact 2R3 and timer switch T9, and a timer operated contact T8, with conductor 66 being connected to one terminal of valve 40. The other terminal of valve 40 is connected to conductor 51 by a conductor 67. One terminal of pilot valve 36 is connected to conductor 66 intermediate contact T8 and contacts 2R3 and T9 by a conductor 70 which has interposed therein a time operated contact T6. The other terminal of valve 36 is connected to conductor 51 by a conductor 71. Thus valve 36 is adapted to be connected across conductors 1th 1 and 11 by conductor 16, start-stop station 13, conductor 66, contacts T9 or 2R3, contact T6 and conductor 70 to the valve and then by conductors 71 and 51 to conductor 11. Ignition transformer 42 has its primary winding connected to conductor 70 by a conductor 72 which has interposed therein a contact T3 operable by the timer T. The other terminal of the primary winding of the ignition transformer is connected by a conductor 73 to conductor 51.

The structure described above is similar to that found in prior burner control apparatus. The additional features provided by the incorporation of my invention in this burner control apparatus is provided by the structure which follows.

Connected between conductors 11 and 111, intermediate the points where conductors 51 and 28 respectively connect thereto, is the series combination of timer operated contacts T12 and normally open contacts 5R2 which are adapted to be operated by a relay 5R which will be described hereinafter.

Connected across conductors and 11 by a conductor 74, and in such position that the circuit thereto is controlled by contacts T12 and 5R2, is the operating winding of an inaccessible disabling relay 5R. Relay 5R has a normally closed contact 5R1 and the normally open contact 5R2 previously described. Relay SR is described as inaccessible in that, as pointed out previously herein, it is not readily resettable and is so imbedded in the control that it can normally be reset only by one skilled in the operation of the control. The normally closed contact 5R1 is interposed in a conductor 75 which is adapted to connect one terminal of an auxiliary alarm 76 to conductor 10. The other terminal of alarm '76 is connected to conductor 51 by a conductor 77. It should be noted that conductor 51 is connected to conductor 11 at such a position that the flow of current through conductor 51 is not controlled by contacts T12 and 5R2.

Interposed in the main fuel supply, downstream of main valve 40 is a pressure responsive device 80 which is responsive to the presence of fuel downstream of the main valve. The device 80 may be of any desired construction such that it will be operable in response to the fluid pressure and, in a preferred form, it may have an operating diaphragm which is responsive to the fluid pressure. Operable by pressure responsive device 80, through appropriate linkage 81, is a normally closed pressure switch 82. One contact of pressure switch 82 is connected to conductor 67 by a conductor 83. The other contact of pressure switch 82 is connected to a conductor 84 having the normally closed 2R4 contact interposed therein. Connector 84 also has interposed therein a timer operated switch T11 and is connected to conductor 29 which, as previously pointed out, is connected to conductor 111. A conductor 85 connects conductor 11, intermediate contacts T12 and 5R2, to conductor 8-4, intermediate contacts 2R4 and T11.

Having described the structure of FIGURE 1, a description of the operation of the apparatus disclosed therein will now be given. As pointed out previously herein, the sequence of operation of the timer contacts is shown in FIGURE 3. For purposes of illustration a typical time cycle has been chosen wherein the complete cycle of operation of the timer includes 120 seconds. This cycle includes a pre-purge period which extends from zero to 40 seconds, a trial for pilot ignition period from 40 to 50 seconds, a main burner ignition period from 50 to 80 seconds, and a run period which extends to 105 seconds at which time the timer operation is interrupted. The timer then is not operated until such time as the demand 8 for operation of the system is satisfied and the main control relay de-energized. At that time the timer operates from the to the second interval which comprises the post-purge period.

As shown in FIGURE 1, switch 12 in conductor -'10 is open and hence no power is supplied to any of the system components. When switch 12 is closed, the system is placed in stand-by condition. The effect of closing switch 12 is the supplying of power to flame detector 46- through conductors e7, 50 and 51 and also the energization of relay 5R. The circuit completed through the coil of relay SR is from conductor 10, through conductor 74 and the relay coil, to conductor 111 and then through conductors 29 and 84, contact T11, conductor 85, and contact T 12 to conductor 11. An alternate path is provided from conductors 74 and 29, through contacts T11 and 2R4 in conductor 84, the closed contacts of switch S2, conductors 83 and 67, and conductor 51 to conductor 11. Energization of relay 5R causes contact 5R1 in conductor 75 to open and hence to interrupt the supply of power to alarm 76. Energization of the 5R relay also causes closing of the 5R2 contacts. This, as stated above, places the control system in a stand-by condition.

When it is desired to operate the system, start switch 14 is closed and this completes a circuit from conductor 10 through conductor 16, start switch 114, closed contact T2, a conductor 86, closed contact 2R1, conductor '17, heater 31, lower portion 29b of the 1R relay coil, conductor 28, contacts 27 and 25, and conductor 29 to conductor 111. This causes energization of relay 1R but, as pointed out previously herein, does not operatively energize heater 31. Energization of relay 1R causes closing of its contacts 1R1, 1R2, and 1R3 and opening of its contact 1R4. Closing of contact 1R1 completes a holding circuit through the coil of relay 1R. This circuit can be traced from conductor 10, through conductor 16, stop switch 15, contacts 1R1, and 2R1, conductor 17, heater 31, portion 29b of the coil of relay 1R, conductor 28, contacts 27 and 25 and conductor 29 to conductor 111. Closing of contact 1R2 in conductor 66 does not immediately cause operation of any of the burner components but simply places this portion of the circuit in condition for subsequent operation. Closing of contact 1R3 completes a circuit through the motor of timer T. This circuit is traced from conductor 10 through the parallel combination of manual switch 57 and timer switch T 4, timer switch T10, contact 13.3, and the motor of timer T to conductor 111. Since contact 1R4 has now opened the connection between conductors 56 and 61) is broken and hence relay 3R is not yet energized.

After about four seconds of operation of the timer, contact T1 closes and completes a circuit from conductor 10 through the contact T1 and conductor 60, and through the coil of relay 3R to conductor 111. This energizes relay SR and closes contact 3R1. This completes the circuit through conductor 61 which connects conductor 10 to one terminal of motor 38. As pointed out previously the other terminal of motor 38 is connected by a conductor 62 to the conductor 51 which is in turn connected to conductor 11. Thus motor 38 is energized and pump 37 and blower 39 are placed in operation. The operation of blower 39 results in a purging of the burner by combustion air and hence provides a pre-purge period. Operation of pump 37 has no effect upon the system since at this time, because valve 41) is closed, the pump simply operates through its by-pass circuit.

At approximately six seconds after the initiation of operation of the timer, timer contact T11 opens. It will be noted by referring to the bar graph of FIGURE 3 that this contact now remains open until just shortly before the end of the post-purge period at the end of the cycle. The purpose of this contact T11 is simply to provide a circuit so that, should the power supply to the system be interrupted during a stand-by period, for example as by the opening of switch 12, the inaccessible relay 5R will be automatically reset upon initiation of the cycle.

9 As was noted previously, de-energization of relay R results in the opening of 5R2 and without the provision of the contact T11 in shunt therewith, there would be no automatic resetting of the 5R relay upon power failure during the stand-by period.

At approximately ten seconds after the initiation of operation of the timer, timer contact T2 opens thus opening the circuit between start switch 14 and conductors 18 and 86. The purpose of this opening of contact T2 is so that operation of the start switch during any subsequent portion of the cycle has no effect upon the burner operation. By referring to FIGURE 3 it will be noted that this Contact also remains open until substantially the end of the complete cycle of the timer.

After about twenty seconds of operation of the timer, contact T12 is opened and remains open until approximately thirty-five seconds of timer operation has elapsed. Since timer contact T11 has already been opened, the only way in which a circuit can be completed from conductor 11 to conductor 111 is through conductors 51, 67, and 83, pressure switch 82, conductor 84, normally closed contact 2R4, conductor 85, and contact 5R2. This provides a safety provision not heretofore found in burner controls. Should valve 40 for some reason be open during this portion of the pre-purge period, the pressure of the fuel would act upon pressure responsive device 80 and switch 82 would be opened thus opening the cir- 'cuit between conductors 11 and 111. This would result in de-energization of relay 1R, relay 5R, relay 2R, timer T, and relay 3R. Thus the system would be disabled and could not be restarted since the de-energization of relay 5R results in the opening of the contact 52R. Upon de-energization of relay 5R, contacts 5R1 close and complete a circuit from conductor through the 5R1 contacts and conductor 75 to alarm '76. This results in the giving of an alarm which indicates that the inaccessible disabling relay has been operated.

Similarly, should the contact 2R4 be opened during this period, as by a false indication of combustion by the flame detector 46 and the consequence energization of relay 2R, or due to welding of the contacts of the relay 2R, the circuit would also be interrupted between conductors 11 and 111 and the 5R relay would be de-energized thus completely disabling the system. As pointed out previously, the system, once disabled in this manner, cannot be readily reset. Resetting requires one who is thoroughly skilled in the operation of the system and who would be aware of the dangerous consequences of the fuel valve being open during this period and/ or the false indication of combustion at this period.

It should be understood that under normal conditions valve 41) will be closed and there will be no indication of combustion during this time and therefore the pressure switch 82 and the contact 2R4 will be closed during this period. If this is true the connection between conductor 11 and conductor 111 will not be interrupted and the system will continue to operate.

Substantially simultaneously with the reclosing of contact T12, that is at approximately thirty-five seconds after initial energization of timer T, timer contact T3 closes preparing ignition transformer 41 for energization. Howver since timer contact T6 is still open, the circuit to the ignition transformer is not yet completed. Also, at approximately thirty-five seconds, timer switch T4 opens. At this time the circuit energizing timer T through conductor 56 is completed only through the manual switch 57. Contact T4 remains open until approximately fortysix seconds of timer operation have elapsed and thus during this period provides a checking period during which the timer may be manually interrupted so that operational adjustments of the system components can be made. After timer switch T 4 is again closed, operation of the manual switch 57 has no effect upon the system.

At about thirty-eight seconds contact T5 is closed by the timer and this completes a circuit through the upper portion 20a of the coil of relay 1R in parallel with heater 31. This results in a transformer effect in this circuit and in heating of the heater 31. Thus at this time the bimetal element 30 begins to warp and the lower end thereof begins to move toward the right and is effective, after a timed delay, if the heater remains energized, to release the contact carrying member 22. Shortly after closing of contact T5, for example at approximately forty seconds after the initiation of the time cycle, timer contact T6 closes. This completes a circuit from conductor 10 through conductor 16, stop switch 15, conductor 66, contacts 1R2, T9, and T6 to pilot valve 36 which is then energized. This circuit is also completed from conductor 70 through contact T3, which has previously closed, and conductor 72 to the primary winding of ignition transformer 42. Thus at this time gas is supplied to the pilot burner 35 and igniter 41 is energized. Normally combustion is established at pilot burner 35 within a very short time after the opening of the pilot valve 36 and the energization of the igniter 41. If this happens, photocell 54 senses flame and the flame detector 46 causes energization of the relay 4R. This causes closing of contact 4R1 and hence completes a circuit through conductor 55 and timer contact T7 so that the winding of relay 2R is connected across conductors 10 and 111 and hence is energized. This results in the opening of contact 2R1 and hence de-energization of heater 31. It will be noted that opening of contact 2R1 does not effect relay 1R since a circuit is still completed through its winding and through conductors 18 and 86. At approximately forty-eight seconds timer contact T7 is opened, and hence, if relay 2R has not been energized prior to that time, it cannot be subsequently energized due to the fact that it is connected between conductors 1t and 11 1 only through its own normally open contact 2R2. If this happens, contact 2R1 will not be opened and heater 31 will continue to heat.

At approximately fifty seconds after initial energization of timer T, timer contact T8 is closed and contact T9 is opened. Opening of contact T9 interrupts the cicuit through conductor 66 unless relay 2R has previously been energized and contact 2R3 closed. Thus, if contact 2R3 is not closed at this time, pilot valve 36 is closed and ignition transformer 42 de-energized. Similarly closing of timer contact T8 does not then complete a circuit to main valve 49 and valve 40 cannot be opened. Timer T will continue to operate and motor 38 remains energized to operate blower 39 which is effective to purge the burner of accumulated, unburned fuel. Since relay 2R has not been energized, contact 2R1 remains closed and theater 30 remains operatively energized until bimetal 30 has warped sufficiently to release member 22. When this occurs, contacts 25 and 27 are separated and the coil of relay 1R is de-energized, opening its contacts 1R1, 1R2, and 1R3 and closing contact 1R4. Closing of contact 1R4 completes a circuit through conductor 60 and the contact 1R4 to the motor of timer T even though contact IRS is open. This allows the burner to continue to be purged of any fuel which may have accumulated there. The timer will then continue to run to the end of its cycle at which time contact T1 opens de-energizing the timer and the relay 3R which in turn de-energizes the motor 40. Operation of safety-switch 21 also results in closing of contacts 24 and 26 which complete a circuit through alarm 62.

From the foregoing, it can be seen that upon failure to establish combustion at the pilot burner during the pilot ignition period, the pilot fuel valve is closed and the main fuel valve is prevented from being energized. Subsequently safety switch 21 operates to de-energize the main control relay 1R and to thereby cause the system to be returned to the stand-by condition and prevent re-cycling thereof. Also it will be seen that, should the circuit through conductor 66 fail to be opened by contacts T9 and 2R3, upon failure to establish combustion during this period, operation of safety-switch 21 will cause de-energization of the fuel valves. Thus safety switch 21 provides an additional safety feature in the control system.

Assuming however that combustion is established at pilot burner 35 during the pilot ignition period, relay 2R will be energized and contact 2R3 will be closed. Thus closure of contact T8 completes a circuit through conductor 66 to main valve 40 and this valve is then energized and fuel supplied to the main burner 34. Under normal conditions combustion is almost immediately established at the main burner. At this time the pilot fuel valve, the main fuel valve, and the ignition transformer are all energized. Also, at this time pressure responsive device 86 will open pressure switch S2 and, since relay ER is energized, contact 2R4 will also be opened. However, since contacts T12 and 5R2 are closed, a circuit is completed between conductors 11 and 111 and the system continues to operate.

Contact T3 is opened at approximately sixty-five seconds after the initiation of timer operation. This deenergizes ignition transformer 42. At approximately eighty seconds contact T6 is opened by the timer T and pilot valve 36 thus de-energized and closed. If combustion has been satisfactorily established at the main burner, relay 4R will be energized by the flame detector and it in turn will maintain contact 4R1 closed so that relay 2R is energized. Therefore at this time the system is under the control of the flame detector 46. At one hundred and five seconds after the initiation of timer, operation contact T opens and since this contact is in series with the motor of timer T, the timer is deenergized and remains de-energized until such time as the main control relay IR is de-energized to close contact 1R4 and hence complete a circuit to the timer motor. This is the run period of the system and during this period the system is completely under the supervision of the flame detector 46. Should the flame detector 46 fail to sense flame at any time during this period, relay 4R would be de-energized and it in turn would cause opening of the circuit to relay 2R. De-energization of relay 2R would in turn open contact 2R3 thus interrupting the circuit to the main fuel valve 40. Also de-energization of relay 2R during this period would result in closing of contact 2R1 and this would complete a circuit through heater 31. Heater 31 would thus act upon bimetal 30 and after a short period of time the bimetal would warp to release member 22, thus opening the contacts 25 and 27 and breaking the circuit to the coil relay 1R.

Assuming that no malfunction occurs during the run period of the system, it continues to operate until the stop button is operated. Since contact T2 is open at this time, operation of stop button 15 interrupts the circuit to the coil of relay 1R. Thus relay IR is deenergized and its contacts 1R1, 1R2, and 1R3 are opened and contact 1R4- is closed. The closing of contact 1R4 completes a circuit to the motor of timer T and it again begins to operate. Opening of contact 1R2 interrupts the circuit through conductor 66 and hence causes deenergization of main fuel valve 40. Since both contact 1R1 and contact T2 are open at this time, operation of start button 14 has no effect upon the system. At this time the timer is re-energized, as pointed out above, and it begins its operation at the one hundred and five second point. Approximately five seconds later contact T8 is opened readying the system for its next cycle of operation and almost immediately thereafter contact T5 is also opened in preparation for the next cycle of operation. At approximately one hundred and eleven seconds contact T10'is closed and almost immediately thereafter contact T9 again closes. The operation of these contacts at this time has no immediate effect upon the system and merely acts to place it in condition for subsequent operation in the next cycle. Very shortly thereafter con tact T7 also is again closed. This too should have no effect upon the system since combustion should have been extinguished at this time and relay 4R de-energized opening contact 4R1 and causing de-energization of relay 2R. At this time the safety provisions of my invention again come into operation. It will be recalled that contact T11 is still open and at approximately one hundred and fifteen seconds after initiation of the cycle contact T12 is again opened by the timer. Again,'as in the checking portion of the pre-purge cycle, a circuit must be completed from conductor 11 through conductor 51 and conductors 67 and 553, through the pressure switch 82 and the contact 2R4 and then through conductor 85 and contact 5R2 to conductor 111 or inaccessible disabling relay 5R will be de-energized. This, as pointed out previously, provides a check to assure that valve 40 has closed and that the flame detector 46 is not indicating the presence of flame at this time. Normally at this time the valve will be closed and the flame detector 46 will have deenergized its relay 4R. If that is the case the circuit is completed between conductors 11 and 111 and relay 5R remains energize-d. At approximately one hundred and eighteen seconds contacts T11 and T12 are again closed. Also during the very last few seconds of the cycle the contact T2 is again closed placing the system in condition for the next cycle of operation.

The system disclosed in FIGURE 2 is similar in construction and operation to that of FIGURE 1 with the exception of the safety feature provided by my invention. In this embodiment, pressure responsive device 80 is replaced by a similar device operating through a linkage 91 to maintain a contact 92 normally open when there is no fuel present downstream of the main valve 40. It will be recalled that device 80 in FIGURE 1 was effective to maintain its pressure switch in a closed position in the absence of fuel and to open the switch contacts when fuel was present downstream of the main valve.

In this embodiment of the invention the inaccessible disabling relay 5R has been deleted and in its place an excess current responsive device such as a fuse or a manually resettable circuit breaker is interposed in conductor 10 intermediate the position where conductors 63 and 16 connect to the conductor 10. This fuse or circuit breaker is made inaccessible so that, as in the case of the inaccessible disabling relay in the embodiment of FIGURE 1, it can be replaced or reset only upon actually getting into the control thus requiring a skilled technician to accomplish replacement or resetting of the device. This device is shown as a fuse indicated at 93 in FIGURE 2 of the drawing. Added to the system is a relay 6R having its winding connected across conductors 10 and 11 by a conductor 94. It will be noted that conductor 94 is connected to conductor 10 as are the conductors 16, 55, 56, and 619, in a position so as to be under the control of the fuse 93.

Connected intermediate conductors 11 and 111 is a timer operated contact T14 which is normally closed and is opened by the timer only during a portion of the run period. As was the case in the circuit of FIGURE 1, one of the contacts of pressure switch $2 is connected to conductor 11 through conductors 83, 67, and 51. The other contact of pressure switch 92 is connected to a conductor 95 which is also connected to conductor 11) intermediate conductors 16 and 55. Interposed in this conductor is a timer operated switch T13 which is adapted to be normally open and to be closed by the timer T during a portion of the pre-purge and post-purge period. A conductor 96 having interposed therein a normally open contact 2R5, operable by secondary flame relay 2R, is adapted to connect conductors 111 and 95 intermediate timer contacts T14 and conductor 29, and intermediate pressure switch 92 and timer contacts T13 respectively.

The operation of the system of FIGURE 2 is identical to that of FIGURE 1 with the exception of the operation of the inaccessible system disabling means. Therefore a 13 complete description of the system of FIGURE 2 will not be given. Upon operation of the start switch 14 the system is placed in operation and it enters a pre-purge period wherein the motor 38 is energized to operate pump 37 and blower 39. During this period the valve 40 will normally be closed and secondary flame relay 2R will be deenergized since no combustion should be taking place at the burner. As pointed out above, during a portion of this period, for example between twenty and thirty-five seconds, timer contact T13 is closed. Also at this time, timer contact T14 is closed. Thus, should the contacts of relay 2R be in a flame position that is with contact 2R5 closed, a circuit is completed from conductor 11 through contacts T14, 2R5, and T13, and conductor 95 to con ductor intermediate conductors 16 and 55. When the i system is connected to a source of voltage, with manual switch 12 closed, this causes a short circuit directly across conductors 1t) and 11 and through the fuse 93. This results in the fuse blowing and opening the circuit of conductor 10 intermediate conductors 63 and 16. Similarly, should the pressure switch 92, which is normally open when no fuel is present downstream of the valve, be open by reason of the valve not being completely closed, a short circuit is also provided through fuse 93 and across the conductors 1i) and 11. This circuit can be traced from conductor 11 through conductor 51, conductors 67 and 83, switch 92, and conductor 95 through the closed contact T13. This again results in operation of the fuse and disabling of the system.

Upon opening of fuse 93 it will be seen that relays 1R, 2R, 3R, and GR, as well as timer T, are de-energized. This results in the system being shut down and deener gization of relay 6R causes closing of contacts 6R1 and hence energization of alarm 76 to give an indication of a malfunction of the system. It will be seen that this operation of the inaccessible system disabling means such as the fuse or circuit breaker 93 takes place either upon the presence of fuel downstream of main valve 4% at pressure responsive device 91), during the portion of the pre-purge period wherein contact T13 is closed or, upon the contacts 2R5 being closed during this period, indicating a malfunction in the secondary flame relay 2R.

Assuming that the system operates normally, pressure switch 92 and contact 2R5 will be opened during this prepurge period and hence fuse 93 will not be operated. If this is the case, the system will go through the remainder of the pre-purge period, the pilot ignition period, the main burner ignition period, and the run period. When the stop-button 15 is operated the system goes into the post-purge period and is returned to the stand-by condition. During a portion of the trial for pilot ignition Pu riod and during the main burner ignition period and the run period, the contact 2R5 will normally be closed because of energization of secondary flame relay 2R and also pressure switch 92 will be closed since during the later two of these three periods since main valve 411 will be open. However this will not result in operation of fuse 93 since contact T13 is open during all three of these periods. During the run period a means for checking the operation of pressure responsive device 99, pressure switch 92, and normally open contact 2R5 is provided. During this period pressure responsive device 90 should be operated to a position indicating the presence of fuel downstream of main valve 40 and hence pressure switch 92 should be closed. Similarly, relay 4R of flame detector 46 should be energized due to the presence of combustion at main burner 34 and consequently secondary flame relay 2R should be energized and contact 2R5 closed. To check the operation of these devices, timer contact T14 is opened during a portion of the run period while the timer is still operating. This may be accomplished, for example, during the ninety to ninety-six second interval. Upon opening of contact T14, a circuit can be completed between conductors 11 and 111 only through pressure switch 92 and contact 2R5. This circuit may be traced from conductor 11 through conductors 51, 67, and 83, pressure switch 92, conductor 95, and contact 2R5. If for some reason pressure switch 92 and contact 2R5 are not closed during this period the operation of the system will be interrupted due to the opening of the circuit between conductors 11 and 111. Thus this feature provides a safety check of the additional safety apparatus which has been provided by the present invention.

Upon operation of stop switch 15 the system enters the post-purge period in the same manner described in connection with FIGURE 1. The timer resumes operation from its one hundred and five second position. After a sufficient time so that combustion at main burner 34 would normally be discontinued, for example at approximately one hundred and fourteen seconds. Contact T13 is again closed for a period of two or three seconds. If, at this time, either pressure switch 92 or contact 2R5 is closed a short circuit is completed from conductor 10 through conductor to conductor 11 in the manner described previously in connection with this checking function during the pre-purge period. Thus if either pressure switch 92 or contacts 2R5 are closed the fuse 93 will be operated to interrupt the circuit of conductor 10 and the system will be disabled. Upon de-energization of relay 6R, contacts 6R1 close and alarm '76 is energized. If the pressure switch and contacts 2R5 are open during this portion of the post-purge period, the system will proceed through the remainder of the post-purge period and return to the stand-by condition.

A modification of the system shown in FIGURE 2 is shown in FIGURE 2a and attention is directed thereto. In this modification the pressure switch 92 has been eliminated and the safety checking feature provided only through the use of the secondary flame relay contact 2R5. In this embodiment conductor 95 has been connected directly to conductor 51 through a timer operated contact T15 which is opened by the timer during a portion of the pre-purge and post-purge periods of the time cycle. In all other respects the system is identical to that described in connection with FIGURE 2.

The operation of the modified structure of FIGURE 2a is as follows. During the pre-purge period of the time cycle, timer contact T15 is open, for example between the twenty and thirty-eighth second interval of this period. Contact T14 is normally closed during this period and, as pointed out in connection with FIGURE 2, contact T13 is closed between the eighteen and thirty-five second interval of this period. Under normal operating conditions contact 2R5 is open during this pre-purge period since combustion should not be present at the burner. If this is the case, with contacts T15 and 2R5 open no circuit is completed through conductor 95 and hence the fuse 93 is not operated. However should the contact 2R5 be closed indicating that secondary flame relay 2R is in a flame position, a circuit is completed from conductor 11, through contacts T14, 2R5, and T13 to provide a short circuit through the fuse 93 and hence the system would be disabled as was described in connection with FIGURE 2. Disabling of the system would again result in deenergization of relay 1R, 2R, 3R, and 6R resulting in a shut down of the system and the de-energization of relay 6R would result in closing of contact 6R1 and hence energization of alarm '76.

Assuming that contact 2R5 is open during this prepurge period, the system proceeds to the trial for pilot ignition and the main burner ignition periods during which contact T13 is opened. Thus closing of contact 2R5 during this period does not result in a circuit being completed through conductor 95. If combustion is established during this time, the system proceeds into the run period and again contact T14, during the period between the ninety and ninety-sixth second interval of the cycle, is opened to insure that the contact 2R5 is closed during that period. With contact T14 open it will be seen that,

in order to keep the system in operation the circuit must be from conductor 11, through conductor 51, contact T15 and conductor 95, conductor 96 and contact 2R5, and then to conductor 111.

When the stop switch 15 is operated the system enters the post-purge period of the cycle. The system again provides a checking function to insure that the main valve has been closed and the 2R relay returned to a no flame position. This is accomplished by the opening of contact T15, for example during the interval of the one hundred and thirteen and one hundred and eighteen seconds. Contact T13, on the other hand, is closed, for example between one hundred and fourteen and one hundred and sixteen seconds. Since contact T14 is closed during the entire post-purge period, should contact 2R5 be closed during the period when contact T13 is closed, a circuit is completed from conductor 11, through contacts T14, 2R5, and T13 to conductor 95 and hence the fuse 93 is short circuited to disable the system. It will be noted that at this time contact 2R5 may be closeed for two reasons. The first of these is that the valve as has not closed as it should have and hence combustion continues at the main burner 34 and flame detector 4-6 is actually indicating the presence of flame. The other is that combustion has discontinued at main burner 34 but for some reason second ary flame relay 2R has remained in the flame position. Thus it will be seen that the system provides a check at this point to determine either a malfunction whereby the valve 40 has not closed or a malfunction in the secondary flame relay ZR.

From the foregoing it can be seen that my invention provides an important safety feature not found heretofore in burner control apparatus. This safety feature lies in the provision of the means for detecting the presence of fuel downstream of the main valve during a portion of the cycle of operation of the system when the valve should be closed and hence no fuel present at the main burner and for completely disabling the system by an inaccessible system disabling means upon occurrence of such malfunction so that the system cannot readily be restarted except by one completely skilled in the operation of the system and familiar with the hazards which result from such a malfunction. In addition the safety feature provides for similar disabling of the system upon a malfunction of the flame or combustion sensing apparatus wherein it is in a flame position during a portion of the cycle, for example during either of the purge periods, wherein no combustion should be present at the burner. Upon operation of the inaccessible system disabling means, separate alarm means are provided for calling attention to the malfunction and the disabling of the system.

Obviously various modifications may be made to the systems described herein without departing from the scope and spirit of the invention. For example the system may utilize the pressure responsive switch alone without its being incorporated with the secondary flame relay contact or, conversely, either of the systems disclosed in FIG- URES 1 and 2 may utilize only the secondary flame relay contact without the pressure switch as exemplified by the structure shown in FIGURE 2a. In addition my invention may be utilized in systems having only one purge period rather than both the pre-purge and post-purge periods as described above. Further, while the pressure responsive device has been described as being located in the main fuel supply line downstream of the main valve 4a, in some instances it may be desirable to place this pressure responsive apparatus intermediate the pump and the main fuel valve. For example, in some systems utilizing a pump with a bypass such as pump 37 described above, more effective operation of the pressure responsive device may result by placing it intermediate the pump and the main valve due to the fact that when the pump is operated with the main valve closed, as during the prepurge or post-purge periods, a substantial pressure builds up upstream of the main valve and, when the main valve is opened this pressure drops off substantially. It is believed that these and other modifications will become apparent to those skilled in the art without departing from the scope of the invention. Therefore I do not intend to be limited to the specific embodiments described herein, but intend that they should be by way of example only. Therefore my invention is to be limited only by the scope of the appended claims.

I claim as my invention:

1. Control apparatus for a fuel burner system of the type including a burner, a blower, ignition means, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus com prising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one purge period; a control relay having an energized position and a de-energized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and including flame switch means operable from a first to a second position when combustion is sensed by said combustion sensing means; means operable by said control relay upon energization thereof for initiating operation of said timer means; means including means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; manually resettable safety lockout means including normally closed switch means in circuit with said control relay; means including means operable by said flame relay and said timer means for rendering said safety lockout means operative to open said normally closed switch means upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; pressure responsive means associated with said fuel supply means and responsive to the presence of fuel downstream of said valve; a normally open pressure switch operable by said pressure reponsive means and adapted to be closed thereby when fuel is present downstream of said valve; inaccessible system disabling means including an inaccessible excessive current responsive, circuit interrupting means; means connecting said circuit interrupting means in the circuit means which is adapted to connect said control relay to a source of power; circuit means for connecting said circuit interrupting means directly across the source of power to cause a short circuit therethrough and to thereby operate said interrupting means to open the circuit to said control relay in the presence of fuel downstream of the valve during said purge period or of said flame switch means being in said second position during the purge period, said circuit means including first circuit means in series with said interrupting means and including in series said normally open pressure switch and a normally open switch operable by said timer to close only during a portion of said purge period, second circuit means connecting a normally open flame switch in shunt with said pressure switch; third circuit means electrically connecting said control relay in shunt with said pressure switch and said last named flame switch and including a switch operable by said timer to open only during a portion of said run period to thereby effect interruption of the circuit to said control relay and shutdown of the system upon failure of said pressure switch and said normally open flame switch to be closed during said run period; second electrically energizable alarm m and me ns pted o connect said second alarm means to a source of power upon operation of said circuit interrupting means.

2. Control apparatus for a fuel burner system of the type including a burner, :a blower, ignition means, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus comprising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one purge period; a control relay having an energized position and a de-energized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and including flame switch means operable from a first to a second position when combustion is sensed by said combustion sensing means; means operable by said control relay upon energization thereof for initiating operation of said timer means; means including means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; manually resettable safety lockout means including normally closed switch means in circuit with said control relay; means including means operable by said flame relay and said timer means for rendering said safety lockout means operative to open said normally closed switch means upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; inaccessible system disabling means including an inaccessible excessive current responsive, circuit interrupting means; means connecting said circuit interrupting means in the circuit means which is adapted to connect said control relay to a source of power; means for connecting said circuit interrupting means directly across the source of power to cause a short circuit therethrough and to thereby operate said interrupting means to open the circuit to said control upon said flame switch means being in said second position during the purge period, said last named means including first circuit means in series with said interrupting means and including a first time switch operable by said timer to close only during a portion of said purge period; a second time switch operable by said timer to open when said first time switch is closed and to close when said first switch is open; second circuit means connecting said second time switch in series with said first switch and intermediate said first switch and the source of power; third circuit means connecting said normally open flame switch in shunt with said second time switch; fourth circuit means electrically connecting said control relay to said second circuit intermediate said timer operated switch and said flame switch; second electrically energizable alarm means; and means adapted to connect said second alarm means to a source of power upon operation of said circuit interrupting means; a first timer switch operable by said timer to close only during a portion of said purge period; a second timer switch operable by said timer to open only when said first timer switch is closed; first circuit means connecting said first and second timer switches in series with said interrupting means with said second timer connected intermediate said first timer switch and the source of power; a normally open flame switch; and second circuit means connecting said last named flame switch between said first circuit means, intermediate said first and second timer switches and the circuit adapted to connect said control relay to the source of power.

3. Control apparatus for a fuel burner system of the type including a burner, a blower, ignition means, and fuel supply means including valve means for controlling the fiow of fuel to the burner, the control apparatus comprising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one purge period; a control relay having an energized position and a de-encrgized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and including flame switch means operable from a first to a second position when combustion is sensed by said combustion sensing means; means operable by said control relay upon energization thereof for initiating operation of said timer means; means including means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; manually resettable safety lockout means including normally closed switch means in circuit with said control relay; means including means operable by said flame relay and said timer means for rendering said safety lockout means operative to open said normally closed switch means upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; pressure responsive means associated with said fuel supply means and responsive to the presence of fuel downstream of said Valve; a normally open pressure switch operable by said pressure responsive means and adapted to be closed thereby when fuel is present downstream of said valve; inaccessible system disabling means including an inaccessible excessive current responsive, circuit interrupting means; means connecting said circuit interrupting means in the circuit means which is adapted to connect said control relay to a source of power; means for connecting said circuit interrupting means directly across the source of power to cause a short circuit therethrough and to thereby operate said interrupting means to open the circuit to said control relay in the presence of fuel downstream of the valve during the purge period, said last named means including a normally open timer switch operable by said timer to close only during a portion of said purge period, and circuit means connecting timer switch and said pressure switch in series with said interrupting means; second electrically energizable alarm means; and means adapted to connect said second alarm means to a source of power upon operation of said circuit interrupting means.

4. In a control apparatus fora fuel burner system of the type including a burner, a blower, an igniter, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus comprising: combustion responsive means; a program timer for sequencing the operation of the burner system upon demand for operation of the system to provide a pre-purge period wherein the blower is operated to purge the burner of unburned fuel, a trial for ignition period wherein the valve means is opened to supply fuel to the burner and the igniter is operated to establish combustion at the burner, a run period wherein the system is placed under the control of the combustion responsive means and the duration of which is normally dependent upon the period of demand for operation of the system and at the end of which the valve means is closed to terminate combustion, and a post-purge period wherein the blower is operated to purge the burner of unburned fuel; manually resetta-ble safety lockout means operable to disable said system; means including means operably by said combustion responsive means for operating said safety lockout means upon failure to establish combustion during said ignition period and upon failure of said combustion responsive means to indicate the presence of combustion during said run period; pressure responsive means associated with the fuel supply means and operable in response to the presence of fuel downstream of the valve means; normally inoperative inaccessible system disabling means operable to disable said system and to prevent restarting thereof; means operable by said pressure responsive means to render said disabling means operative in the presence of fuel downstream of the valve; means operable by said combustion responsive means for rendering said disabling means operative when said combustion responsive means indicates the presence of combustion at the burner; and means operable by said timer to render said last two named means effective to render said disabling means operative only during said pre-purge period and said postpurge period.

5. Control apparatus for a fuel burner system of the type including a burner, a blower, ignition means, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus comprising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one urge period; a control relay having an energized position and a deenergized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and including flame switch means operable from a first to a second position when combustion is sensed by said combustion sensing means; means operable by said control relay upon energization thereof for initiating operation of said timer means; means including means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; means operable by said timer means and said control relay for interrupting operation of said timer means during said run period and for-reenergizing said timer means when said control relay is de-energized; manually resettable safety lockout means including normally closed switch means in circuit with said control relay; means including means operable by said flame relay and said timer means for rendering said safety lockout means operative to open said normally closed switch means upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; pressure responsive means associated with said fuel supply means responsive to the presence of fuel downstream of said valve; a normally closed pressure switch operable by said pressure responsive means and adapted to be opened thereby when fuel is present downstream of said valve; inaccessible system disabling means including an inaccessible relay having a winding; a normally open relay switch and a normally closed relay switch each operable by said winding upon energization thereof; circuit means adapted to connect said relay winding to a source of power and electrically in parallel with said control relay, said circuit means including, in series with said control relay and said inaccessible relay winding, first circuit means including a normally closed switch operable by said timer to open during a portion of said purge period, second circuit means in shunt with said first circuit means and including, in series connection, said pressure switch and a normally open flame switch, third circuit means electrically connecting in parallel said normally open relay switch and a normally open switch operable by said timer to close only during a portion of said purge period when said switch in said first circuit 13 closed; fourth circuit means electrically connecting the parallel combination of said first and second circuit means in series with said third circuit means; second electrically energizable alarm means; and circuit means including sa d normally closed relay switch and adapted to connect said second alarm means to a source of power.

6. In combination with a fuel burner system including a burner, a blower, fuel supply means including valve means upstream of the burner for controlling the flow of fluid thereto, and an igniter, control apparatus for the sys tem comprising: an electrically energizable control relay for controlling operation of the system; circuit means adapted to connect said control relay to a source of power through a control device to energize said control relay upon demand for operation of the burner system; flame sensing means including a flame relay adapted to be energized upon sensing of combustion at said burner; timer means adapted to provide a sequence of operation of the system including an ignition period, a run period, and at least one purge period; means operable upon energization of said control relay to initiate operation of said timer means; manually resettable safety lockout means operable to interrupt the circuit to said control relay and to thereby interrupt operation of the system; means operable by said timer means and said flame relay to operate said safety lockout means upon failure to establish combustion during said ignition period and upon failure of said flame sensing means to indicate the presence of flame during the run period; pressure responsive means associated with the fuel supply means and operable by the pressure of fuel in the system when the valve means is open; an inaccessible electrically energizable lockout relay having a normally open switch; means connecting said normally open switch in series circuit with said control relay; circuit means adapted to connect said lockout relay to a source of power and including, in series, first normally closed switch means operable by said pressure responsive means and adapted to be opened by said pressure responsive means only when said fuel valve means is open, second normally closed switch means operable by said flame relay and adapted to be opened upon energization of said flame relay, and said normally open switch of the lockout relay; and a timer operated switch opened by said timer only during the purge period and connected in shunt with said first and second normally closed switch means so that said lockout relay is energized at all times except when the fuel valve is open during a purge period and When the flame sensing means indicates the presence of combustion during a purge period.

7. Control apparatus for a fuel burner system of the type including a burner, a blower, ignition means, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus comprising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one purge period; a control relay having an energized position and a deenergized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and including flame switch means openable from a first to a second position when combustion is sensed by said combustion sensing means; means operable by said control relay upon energization thereof for initiating operation of said timer means; means in cluding means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; means operable by said timer means and said control relay for interrupting operation of said timer means during said run period and for re-energizing said timer means when said control relay is deenergized; manually resettable safety lockout means including normally closed switch means in circuit with said control relay; means including means operable by said flame relay and said timer means for rendering said safety lockout means operative to open said normally closed switch means upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; pressure responsive means associated with said fuel supply means responsive to the presence of fuel downstream of said valve; a normally closed pressure switch operable by said pressure responsive means and adapted to be opened thereby when fuel is present downstream of said valve; inaccessible system disabling means for disabling said system and preventing restarting thereof and including an inaccessible relay having a winding; a normally open relay switch and a normally closed relay switch each operable by said winding upon energization thereof; circuit means adapted to connect said relay winding to a source of power and electrically in parallel with said control relay, said circuit means including, in series with said control relay and said inaccessible relay winding, first circuit means including a normally closed switch operable by said timer to open during a portion of said purge period, second circuit means connecting said pressure switch in shunt with said first circuit means, third circuit means electrically connecting in parallel said normally open relay switch and a normally open switch operable by said timer to close only during a portion of said purge period when said switch in said first circuit is closed; fourth circuit means electrically connecting the parallel combination of said first and second circuit means in series with said third circuit means; second electrically energizable alarm means; and circuit means including said normally closed relay switch and adapted to connect said second alarm means to a source of power.

8. Control apparatus for a fuel burner system of the type including a burner, a blower, ignition means, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus comprising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one purge period; a control relay having an energized position and a de-energized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and including flame switch means operable from a first to a second position when combustion is sensed by said combustion sensing means; means operable by said control relay upon energization thereof for initiating operation of said timer means; means including means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; means operable by said timer means and said control relay for interrupting operation of said timer means during said run period and for reenergizing said timer means when said control relay is deenergized; manually resettable safety lock-out means including normally closed switch means in circuit with said control relay; means including means operable by said flame relay and said timer means for rendering said safety lockout means operative to open said normally closed switch means upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; inaccessible system disabling means for disabling said system and preventing restarting thereof when said flame switch means is in said second position during a portion of said purge period, said disabling means including an inac cessible relay having a winding; a normally open relay switch and a normally closed relay switch each operable by said winding upon energization thereof; a pair of flame switch contacts which are closed when said flame switch means is in said first position and open when said flame switch is in said second position, circuit means adapted to connect said relay winding to a source of power and electrically in parallel with said control relay, said circuit means including, in series with said control relay and said inaccessible relay winding, first circuit means including a normally closed switch operable by said timer to open during a portion of said purge period, second circuit means connecting said flame switch contacts in shunt with said first circuit means, third circuit means electrically connecting in parallel said normally open relay switch and a normally open switch operable by said timer to close only during a portion of said purge period when said switch in said first circuit is closed; fourth circuit means electrically connecting the parallel combination of said first and second circuit means in series with said third circuit means; second electrically energizable alarm means; and circuit means including said normally closed relay switch and adapted to connect said second alarm means to a source of power.

9. Control apparatus for a fuel burner system of the type including a burner, a blower, ignition means, and fuel supply means including valve means for controlling the flow of fuel to the burner, the control apparatus comprising: a timer operable to define a time cycle of operation of the burner components including an ignition period, a run period, and at least one purge period; a control relay having an energized position and a deenergized position and adapted to be energized upon demand for operation of the burner system; circuit means adapted to connect said control relay to a source of power through a control device operable in response to a demand for operation of the burner system; combustion sensing means; a flame relay associated with said combustion sensing means and adapted to be energized upon said combustion sensing means indicating the presence of combustion; means operable by said control relay upon energization thereof for initiating operation of said timer means; means including means operable by said timer means and said control relay for energizing the valve means and the ignition means during said ignition period; means including means operable by said control relay and said flame relay for maintaining said valve means energized during said run period; means including means operable by said timer means and said control relay for interrupting operation of said timer means during said run period and for again rendering said timer means operable upon de-energization of said control relay; manually resettable safety lockout means operable to de-energize said control relay; means including means operable by said flame relay and said timer means for rendering said safety' lockout means operative upon failure to establish combustion during said ignition period or upon failure of said combustion sensing means to indicate the presence of combustion during said run period; first alarm means operable upon operation of said safety lockout means; inaccessible system disabling means operable to de-energize said control relay and thereby shut down said system and to prevent restarting thereof; pressure responsibe means associ-.

ated with said fuel supply means and responsive to the presence of fuel downstream of the valve means; means operable by said pressure responsive means to operate said system disabling means when there is fuel present downstream of said valve means; means operable when said flame relay is in energized condition to operate said system disabling means; means operable by said timer means and rendering said system disabling means operable only during said purge period; second alarm means associated with said system disabling means; and means for operating said second alarm means when said system disabling means is operated.

10. In a control apparatus for a fuel burner system of the type having a valve for controlling the flow of fuel to the burner and wherein the control apparatus includes a timer operable to define a time cycle of operation for the burner components including an ignition period, a run period, and at least one purge period, manually resettable safety cut-out means rendered operable by said timer and effective to interrupt operation of said system a fixed time after the initiation of said ignition period, combustion sensing means responsive to combustion at the burner, said combustion sensing means including flame switch means operable from a first to a second position by the combustion sensing means when combustion is sensed at the burner, means operable by said flame switch means to render said safety cut-out means inoperative upon the establishment of combustion during the ignition period, the improvement comprising: inaccessible system disabling means operable to terminate the operation of said system and to prevent restarting thereof; means including the flame switch means of said combustion sensing means effective to operate said system disabling means when said flame switch means is in its second position; means operable by said timer and in controlling relationship with said system disabling means, said last named means rendering said system disabling means operable only during said purge period; and alarm means rendered operable when said system disabling means is operated.

11. The control apparatus of claim wherein the improvement further comprises: pressure responsive means associated with the fuel valve and including means independent of said flame switch means for operating said system disabling means when the fuel valve is open.

12. Burner control apparatus for controlling a burner system of the type including a burner, a fuel supply system including a valve upstream of the burner and a blower, the control apparatus comprising: a control relay adapted to be energized upon demand for operation of the system and in controlling relationship with said system; a timer for sequencing the operation of said burner components through a time cycle including at least one purge period, an ignition period and a run period; a flame relay responsive to combustion at said burner and including flame switch means operable between flame and no flame" positions; manually resettable safety lockout means and operable to cause de-energization of said control relay upon failure to establish combustion during said ignition period or upon said flame switch means occupying the no flame position during the run cycle; inaccessible safety lockout means operable to disable said system and prevent restarting thereof; pressure operated means responsive to the presence of fuel downstream of the supply valve; means placing said pressure responsive means in controlling relationship with said inaccessible lockout means for operating said disabling means when fuel is present downstream of the valve; means placing said flame relay in controlling relationship with said inaccessible lockout means for operation thereof when said flame switch means is in said flame position; and means operable by said timer and in overriding controlling relationship with said inaccessible lockout means and ren dering said inaccessible lockout means operable only during said purge period.

13. Control apparatus for a fuel burner system of the type including a burner, a blower, fuel supply means including a valve for controlling the flow of fuel to the burner, and igniter means for igniting fuel to establish combustion at the burner, the control apparatus comprising: a program timer to provide a given sequence of operation of the burner system including a pre-purge period wherein the blower is operated to purge the burner of unburned fuel, an ignition period wherein combustion is established at the burner, a run period during which the system is placed under control of said combustion sensing means, and a post purge period wherein the blower alone is operated to purge the burner of unburned fuel; safety lockout means effective to disable said systern upon failure to establish combustion during the trial for ignition period and upon failure of combustion during said run period, means responsive to the presence of fuel downstream of the valve; inaccessible system disabling means operable to disable said system and prevent restarting thereof; means operable by said means responsive to the presence of fuel downstream of the valve to operate said system disabling means and thereby disable said system; and means operable by said timer and effective to render said system disabling means operable only during said pre-purge period and said post-purge period of the operating sequence.

14. In a control apparatus for a fuel burner system of the type including a burner, fuel supply means including a valve for controlling the flow of fuel to the burner, and an igniter for igniting fuel at the burner, and the control apparatus including a program timer to provide a sequence of operation of the burner system including a pre-purge period, an ignition period, a run period, and a post-purge period, combustion sensing means, and resettable safety switch means for disabling the system upon failure to establish combustion during the trial for ignition period and upon failure of said combustion sensing means to indicate the presence of combustion during said run period, the improvement comprising: means associated with said fuel supply means and responsive to fuel pressure therein, said pressure responsive means being operable between a first control position when the fuel supply valve is open and a second control position when the valve is closed; inaccessible system disabling means; and means including switch means operable by said pressure responsive means and by said timer to operate said system disabling means only when said pressure responsive means is in said first control position during the pre-purge period or the postpurge period of the sequence of operation to thereby disable the system and prevent resetting thereof when the fuel valve is open during either of said periods.

15. In a fuel burner control system of the type having combustion sensing means, a timer providing a sequence of operations including an ignition period, a run period, and at least one purge period, and manually resettable safety lockout means rendered effective by said timer and said combustion sensing means to disable the system upon failure of the combustion sensing means to indicate the presence of combustion during the run period, the improvement comprising: inaccessible system disabling means operable to disable the system and to prevent restarting thereof; fuel pressure responsive means for operating said system disabling means when fuel is supplied to the burner; means operable by said combustion sensing means for operating said disabling means when said combustion sensing means indicates the presence of cornbustion; and means operable by said timer and rendering said disabling means operable only during said purge period.

16. In an electrically operated control apparatus for a fuel burner system of the type including a burner, a blower, fuel supply means including a valve for controlling the flow of fuel to the burner, and igniter means for igniting fuel to establish combustion at the burner, the control apparatus comprising: circuit means adapted to be connected to a source of power and to supply power therefrom to the control apparatus; a program timer to provide a given sequence of operation of the burner system including a pre-purge period wherein the blower is perated to purge the burner of unburned fuel, an ignition period wherein combustion is established at the burner, 21 run period during which the system is placed under control of said combustion sensing means, and a post purge period wherein the blower alone is operated to purge the burner of unburned fuel; manually resettable safety lockout means effective to disable said system upon failure to establish combustion during the trial for ignition period and upon failure of combustion during said run period; means responsive to the presence of fuel downstream of the valve; inaccessible system disabling means operable to disable said system and prevent restarting thereof; said disabling means including an inaccessible relay having a coil adapted to be connected to a source of power by said circuit means and first switch means maintained closed when said coil is energized and opened when said coil is de-energized; second switch means normally closed and operable by said means responsive to the presence of fuel downstream of the valve to open when fuel is present downstream of the valve; means connecting said first and second switch means in said circuit means in controlling relationship with said control apparatus including said relay coil; third normally closed switch means operable by said timer and opened thereby only during a portion of said pre-purge and post-purge periods; and means connecting said third switch means in shunt with said second switch means.

17. In an electrically operated control apparatus for a fuel burner system of the type including a burner, a blower, fuel supply means including a valve for controlling the flow of fuel to the burner, and igniter means for ig niting fuel to establish combustion at the burner, the control apparatus comprising: circuit means adapted to be connected to a source of power and to supply power therefrom to the control apparatus; a program timer to provide a given sequence of operation of the burner system including a pre-purge period wherein the blower is operated to purge the burner of unburned fuel, an ignition period wherein combustion is established at the burner, a run period during which the system is placed under control of said combustion sensing means, and a post purge period wherein the blower alone is operated to purge the burner of unburned fuel; manually resettable safety lockout means effective to disable said system upon failure to establish combustion during the trial for ignition period and upon failure of combustion during said run period, inaccessible system disabling means operable to disable said system and prevent restarting thereof, said disabling means including an inaccessible, excessive current responsive, circuit interrupting means; means connecting said disabling means in said circuit means so that upon operation of said disabling means the power supply to the control apparatus is interrupted, said disabling means having characteristics such that normal operation of the system does not result in operation of said disabling means; means responsive to the presence of fuel downstream of the valve; first normally open switch means operable by said last named means to open when fuel is present downstream of the valve; second normally open switch means operable by said timer and closed thereby only during a portion of said pre-purge and post-purge periods; and further circuit means connecting said first and second switch means and said disabling means electrically in series and adapted to be connected directly across a source of power so that upon simultaneous closing of said first and second switch means said disabling means is operated to interrupt the supply of power to the control apparatus.

References Cited by the Examiner UNITED STATES PATENTS 3,194,296 7/1965 Brown 158-122 JAMES W. WESTHAVER, Primary Examiner. 

15. IN A FUEL BURNER CONTROL SYSTEM OF THE TYPE HAVING COMBUSTION SENSING MEANS, A TIMER PROVIDING A SEQUENCE OF OPERATIONS INCLUDING A IGNTION PERIOD, A RUN PERIOD, AND AT LEAST ONE PURGE PERIOD, AND MANUALLY RESETTABLE SAFETY LOCKOUT MEANS RENDERED EFFECTIVELY BY SAID TIMER AND SAID COMBUSTION SENSING MEANS TO DISABLE THE SYSTEM UPON FAILURE OF THE COMBUSTION SENSING MEANS TO INDICATE THE PRESENCE OF COMBINATION DURING THE RUN PERIOD, THE IMPROVEMENT COMPISING: INACESSIBLE SYSTEM DISABLING MEANS OPERABLE TOI DISABLE THE SYSTEM AND TO PREVENT RESTARTING THERETO; FUEL PRESSURE RESPONSIVE MEANS FOR OPERATING SAID SYSTEM DISABLING MEANS WHEN FUEL IS SUPPLIED TO THE BURNER; MEANS OPERABLE BY SAID COMBUSTION SENSING MEANS FOR OPERATING SAID DISABLING MEANS WHEN SAID COMBUSTION SENSING MEANS INDICATES THE PRESENCE OF COMBUSTION; AND MEANS OPEABLE BY SAID TIMER AND RENDERING SAID DISABLING MEANS OPERABLE ONLY DURING SAID PURGE PERIOD. 